Commonly owned U.S. Pat. No. 4,347,608 discloses a selfchecking system for electronic processing equipment in which a central processor controls a set of peripheral units through an associated logic network. The processor is programmed to activate from time to time, through a direct connection which bypasses the logic network, a checking unit wherein a read-only memory stores various microprograms in areas individually addressable by the processor. Thus, the processor monitors the operation of the checking unit in the course of a diagnostic program during which the performance of the logic network is also tested.
In many instances, terminals or peripheral units connected thereto have their own time bases enabling each terminal to transmit and/or receive data at least once during a predetermined recurrence period. An interface such as the aforementioned logic network must then be able to handle the periodically recurring access requests from the several terminals, arriving over the common synchronous data bus, along with similar but irregularly occurring requests coming from the asynchronous bus associated with the processing unit whose main program may or may not be of the diagnostic type discussed in the prior patent. An important task of the interface, therefore, is to avoid collisions between access requests from different sources and to allocate time for the data transfer which is to follow such a request.
If the recurrence periods of the access requests from the several terminals are mutually different, the shortest period should still be sufficient for a data transmission to or from each terminal over the common synchronous bus, as well as from or to the processor on the asynchronous bus, while also allowing part of the main program to be executed. Usually, though, the several time bases are provided with crystal-controlled oscillators tuned to the same nominal frequency so that their various recurrence periods are substantially identical. In theory, therefore, it is possible to stagger these recurrence periods so as to prevent a coincidence of the access requests which are in the form of short pulses whose duration is a small fraction of such a period. However, mutually independent oscillators cannot indefinitely maintain such a staggered relationship but are subject to relative frequency drifts due, for example, to thermal causes. In certain situations, as where the terminals form part of a message-transmitting system using a communications satellite, Doppler shifts will also be a factor; especially in such a communication system the recurrence period may be on the order of 500 .mu.s or less, allowing only a minor fraction of that time for data transfer from or to a given terminal even if only two terminals are connected to the common synchronous bus.